Self-propelled pneumatic tools for making small diameter holes through soil are well known. Such tools are used to form holes for pipes or cables beneath roadways without need for digging a trench across the roadway. These tools include, as general components, a torpedo-shaped body having a tapered nose and an open rear end, an air supply hose which enters the rear of the tool and connects it to an air compressor, a piston or striker disposed for reciprocal movement within the tool, and an air distributing mechanism for causing the striker to move rapidly back and forth. The striker impacts against the front wall (anvil) of the interior of the tool body, causing the tool to move violently forward into the soil. The friction between the outside of the tool body and the surrounding soil tends to hold the tool in place as the striker moves back for another blow, resulting in incremental forward movement through the soil. Exhaust passages are provided in the tail assembly of the tool to allow spent compressed air to escape into the atmosphere.
Most impact boring tools of this type have a valveless air distributing mechanism which utilizes a stepped air inlet. See, for example, Wentworth et al. U.S. Pat. Nos. 5,025,868 and 5,199,151. The step of the air inlet is in sliding, sealing contact with a tubular cavity in the rear of the striker. The striker has radial passages through the tubular wall surrounding this cavity, and an outer bearing surface of enlarged diameter at the rear end of the striker. This bearing surface engages the inner surface of the tool body.
Air fed into the tool enters the cavity in the striker through the air inlet, creating a constant pressure which urges the striker forward. When the striker has moved forward sufficiently far so that the radial passages clear the front end of the step, compressed air enters the space between the striker and the body ahead of the bearing surface at the rear of the striker. Since the cross-sectional area of the front of the striker is greater than the cross-sectional area of its rear cavity, the net force exerted by the compressed air now urges the striker backwards instead of forwards. This generally happens just after the striker has imparted a blow to the anvil at the front of the tool.
As the striker moves rearwardly, the radial holes pass back over the step and isolate the front chamber of the tool from the compressed air supply. The momentum of the striker carries it rearwardly until the radial holes clear the rear end of the step. At this time the pressure in the front chamber is relieved because the air therein rushes out through the radial holes and passes through exhaust passages at the rear of the tool into the atmosphere. The pressure in the rear cavity of the striker, which defines a constant pressure chamber together with the stepped air inlet, then causes the striker to move forwardly again, and the cycle is repeated.
These tools have been made reversible by providing a threaded connection between the air inlet sleeve and the surrounding structure which holds the air inlet concentric with the tool body. The threaded connection allows the operator to rotate the air supply hose and thereby displace the stepped air inlet rearwardly relative to the striker. Since the stroke of the striker is determined by the position of the step, i.e., the positions at which the radial holes are uncovered, rearward displacement of the stepped air inlet causes the striker to hit against the tail nut at the rear of the tool instead of the front anvil, driving the tool rearwardly out of the hole.
Impact tools of this type have been used in the installation of underground pipes. One such method is described in Watts Jr. et al. U.S. Pat. No. 4,067,200, wherein a flexible duct is pulled along behind a soil penetrating device as it moves through the soil. This method works well for installation of lightweight flexible pipes, but is less effective when used on heavier steel pipes.
To install steel pipes, pneumatic impact tools of the general type shown in the foregoing patents have been used to push a pipe into the ground rather than pull it. A trench is dug in the direction of insertion, and the tool is positioned in the far end of the trench. An adapter is fitted over the tool nose and positioned in the pipe. The pipe is placed in the near end of the trench with its front end in engagement with the trench wall at the installation site. The tool is then operated to pound the pipe into the ground end first. As the pipe moves into the ground, the tool follows, moving along towards the near end of the trench. See generally Bouplon U.S. Pat. No. 4,329,077, Schmidt U.S. Pat. Nos. 4,671,703 and 4,650,374, and Total Quality Systems, TT Technologies, 1991 at pages 13-15.
When the pipe has been fully inserted, the tool is operated in reverse to disengage the tool from the adapter, and the adapter is then removed from inside the pipe. A second section of pipe is then welded to the exposed end of the first pipe, and the adapter and tool are reinserted into the far end of the second pipe section. The process is then repeated as many times as needed to complete the run.
Once the end of the first pipe has emerged at the target site at the end of the run, the tool and adapter are removed from the trench, and one end of the pipe is capped with a fitting including a high pressure nozzle. Soil inside the pipe is then removed by first flooding the interior of the pipe with water through the nozzle and then forcing the soil out using compressed air in a manner well known in the art.